Well, shit. I'll try to keep my explanation as concise as possible, but I could honestly write a few pages to answer it. Master Chief is the luckiest soldier in the history of video games. He somehow managed to never die when falling from low orbit, defeated a forerunner that could easily have squeezed his brains out, escaped multiple locations that were about to explode ( Pillar of Autumn, Installation 4B)... the list goes on. In addition, Master Chief has the most intensive training known in the Halo universe. He was taught every military strategy known to man as a child, and he was mentally and emotionally strengthened by effectively becoming a soldier at the age of 6. Furthermore, he has served in various military operations since his adolescence. And don't forget his augmentations- his bones are nearly unbreakable, his reflexes are tripled by altered neural dendrites, and a thyroid pellet stimulates muscle growth. Those aren't even all of his augmentations, either- they're the ones I could think of off the top of my head. And then Mjolnir further improves on those already insane characteristics. Chief's success can be represented by a conflict with ODSTs when he was 14. Before his augmentations had taken full effect, and without any armor or weapons, he singlehandedly killed three ODSTs and critically wounded another when they tried to beat him up. Now, the Flood are even more ridiculous. They were created from the remains of Precursors, who basically created all life in the galaxy and made indestructible objects on a huge scale (ie star roads). While the Flood cannot recreate the work of the Precursors, they have all Precursor knowledge and are linked by a Gravemind, pretty much making the Flood one big intelligent lifeform (think of it like the Vex). The Flood are capable of infecting entire planets within hours, and they infected the entire Orion arm of the galaxy during the forerunner-flood war. Plus, their hosts retain all previous abilities, from what we have seen so far- infected humans and elites can still fire guns and such. So, yeah, Halo is OP af. Edit: I forgot to mention that Chief's iq is estimated to be around 160, the same as Albert Einstein.

Dewritos

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For chief it all comes down to luck. For the flood it's all about the millennia of evolution and being created from the DNA of a race vastly superior to humanity.

The Chief is insanely luck due to the power of plot devices. Up until 4 he also had the most advanced AI ever created by humans. The Flood have sentient Graveminds and can take control of literally and ship and use it to cook their "food". Their army also grows the more they kill. They were so powerful that the Forerunners had to do a hard reset on the galaxy to stop them.

The flood is a bit complicated, so I'll just start with the chief for now. The Master Cheif isn't special by Spartan standards except for one regard: his astronomical luck. Other than his luck, he isn't the strongest, fastest, smartest, or even the best leader. Sure he is a spartan, so he does have a lot of augmentations and training that does help, but his main advantage and the reason he is "So OP" is because of his luck.